The experiments in this proposal are designed to explore the mechanism of visual adaptation in vertebrate rods. We will be simplifying the rod response by using pharmacological agents known from previous work to block voltage-dependent conductances in rod membrane. In this way we hope to separate effects of light-dependent conductances and voltage-dependent conductances on the adaptive state of rods. We also will be examining the contribution of voltage-dependent conductances to the kinetics of the response waveform during light and dark adaptation. We will investigate the relation, if any, of light adaptation to "fast" or "neural" dark adaptation. We will also look carefully at the relation between sensitivity and pigment concentration during "slow" dark adaptation in single receptors. Finally, we shall examine the role of receptor connections in producing spatial interactions during rod adaptation. The purpose of these experiments is twofold: First, to discover the role of voltage-dependent conductances in the normal physiology of rods: and second, to understand more clearly how visual adaptation occurs. These experiments will also contribute to our basic knowledge about the pharmacology of the retina.